A　novel　free　piston　expander-linear　generator　(FPE-LG)　integrated　unit　was　proposed　to　recover　waste　heat　efficiently　from　vehicle　engine.　This　integrated　unit　can　be　used　in　a　small-scale　Organic　Rankine　Cycle　(ORC)　system　and　can　directly　convert　the　thermodynamic　energy　of　working　fluid　into　electric　energy.　The　conceptual　design　of　the　free　piston　expander　(FPE)　was　introduced　and　discussed.　A　cam　plate　and　the　corresponding　valve　train　were　used　to　control　the　inlet　and　outlet　valve　timing　of　the　FPE.　The　working　principle　of　the　FPE-LG　was　proven　to　be　feasible　using　an　air　test　rig.　The　indicated　efficiency　of　the　FPE　was　obtained　from　the　p-V　indicator　diagram.　The　dynamic　characteristics　of　the　in-cylinder　flow　field　during　the　intake　and　exhaust　processes　of　the　FPE　were　analyzed　based　on　Fluent　software　and　3D　numerical　simulation　models　using　a　computation　fluid　dynamics　method.　Results　show　that　the　indicated　efficiency　of　the　FPE　can　reach　66.2%　and　the　maximal　electric　power　output　of　the　FPE-LG　can　reach　22.7　W　when　the　working　frequency　is　3　Hz　and　intake　pressure　is　0.2　MPa.　Two　large-scale　vortices　are　formed　during　the　intake　process　because　of　the　non-uniform　distribution　of　velocity　and　pressure.　The　vortex　flow　will　convert　pressure　energy　and　kinetic　energy　into　thermodynamic　energy　for　the　working　fluid,　which　weakens　the　power　capacity　of　the　working　fluid.